1. Field of the Invention
The present invention relates to an image forming method and apparatus capable of automatically conveying documents, and more particularly to an image forming method and apparatus capable of duplex (i.e., double-side) scanning at an increased speed and of automatically conveying documents in a convenient manner.
2. Discussion of the Background Arts
In a background image forming apparatus or image scanning apparatus, such as a copier, a printer, and a facsimile machine, various types of document conveying devices are used for conveying documents to be scanned. In a commonly used background document conveying device, a pick-up device such as a pick-up roller picks up documents set on the document conveying device, and one of the documents is separated from remaining documents and conveyed by a separation belt and a reverse roller provided at downstream positions of the pick-up roller in a document conveying direction.
The document thus separated from the remaining documents is conveyed onto a contact glass and scanned. In this background document conveying device, however, there is a time loss before a scanner carriage returns to its scanning start position. This time loss reduces CPM (copies per minute, i.e., the number of copies output in one minute by a copier), compared with in a case in which a printing operation is exclusively performed. Further, to discharge scanned documents in a page order, the documents are discharged on an external tray. As a result, a size of the image forming apparatus is increased.
In light of the above, an image forming apparatus using a sheet-through system of conveying documents over a fixed scanner carriage is expected to become mainstream. According to the system, an interval between one document and its subsequent document can be controlled. If this interval is reduced to the minimum value, therefore, the CPM can be increased up to a CPU value obtained in the printing operation. In addition, the documents are discharged to a document discharging and stacking mechanism provided under a document table on which the documents are set. Therefore, the external tray is unnecessary, and the image forming apparatus can be reduced in size.
In an image forming apparatus such as a copier, for example, productivity of duplex copies, as compared with productivity of simplex (i.e., single-sided) copies, is an important factor, as well as an output speed of the simplex copies. To increase the productivity of the duplex copies, reduction in a time interval between scanning one document and scanning its subsequent document is a key issue.
In a copying operation of a duplex document, after a back surface of the duplex document is scanned, the duplex document needs to be discharged with its front surface facing downward so that discharged documents are arranged in the page order. In a typical sheet-reversing and switchback operation, both surfaces of the duplex document are scanned, and then the duplex document is conveyed once again over a scanning position before being discharged. That is, the duplex document is conveyed over the scanning position not for being scanned but for being reversed. Therefore, the duplex document needs to be reversed three times in total, i.e., for scanning both surfaces of the duplex document and for arranging the duplex document in the page order.
In light of this, there is another background image forming apparatus which increases the productivity of duplex copying operation by combining a sheet-reversing and switchbacking device and a sheet-reversing and discharging device provided at downstream positions of a scanning mechanism, without reversing each duplex document three times. With this configuration, duplex documents are discharged with each of them reversed and arranged in the page order without being conveyed through the scanning mechanism.
To reverse and switch back one document in this image forming apparatus, however, the document needs to be discharged before scanning of its subsequent document starts. Thus, a time interval between scanning of the document and scanning of its subsequent document may not be sufficiently reduced. Further, as a size of a document increases, a longer time is taken before scanning the front surface of its subsequent document starts.